I'm on a quick turn and after the mandatory break to pee, refill water bottles and file a new flight plan I'm back at the ramp to check on the airplane. My fuel caps are recessed into the top of the wings, with flaps that cover them, flush with the wing. The flaps are opened by giving a half-turn to a wingnut. One of the wingnuts is now missing. I track down the fueller. "Did the wingnut break off the fuel cap?" I ask.
He says it was like that when he found it. It seems very odd to me that a part that was affixed well enough for me to have opened and closed the cap to check it just five hours earlier became loose enough to bounce off in flight of its own accord. Sure there's airflow over it, but really? There's no point in arguing. Fortunately when the wingnut broke off it left a screwhead underneath and the cap is still perfectly usable to anyone with a tool. I have a tool.